i would say an underground bunker with a small network of interlinking tunnels like those used by the germans. the under ground base only has a couple of exits that can easily be guarded by at least 15 people and no windows means less trips out of the bunker
You've got to be able to move concrete. You've got to be able to dig deep ditches. This takes heavy equipment that can be a little complicated to use. Most reasonable people can figure out how to work the equipment. Once you figure how to work the equipment, find the supplies and get things moving it takes time to get it done. You had better have started this months before the outbreak because you don't have time when it is at your door. Plus the noise from the machinery would draw the zeds right to you. You need to get up something quick and sturdy. If you're in a city or town then it needs to be a temporary place. Plywood the first floor windows inside and out. Razor wire stretched around the entire building and tripled up. The wire isn't going to stop the zeds but it will slow them down and tangle them up. Remember they feel no pain. An escape route and a plan B escape route.
I agree that bunkers and concrete walls would be best, but like Prime said, you gotta have time to do that shit. So until you can safely build a permanent concrete bunker, I'd say reinforce all the windows with wood. Plywood would probably be easiest but 2bys would be even stronger. If you go with plywood be sure to use 3/4in and put it on both sides of the windows. If your door isn't all that strong then reinforce that too. I would definitely reinforce garage doors.
I would start with a secure warehouse, take down all the fans, weld and wire them together, side by side by side and use it to block off one door and turn it on .With the noise and the smell it will pull from in the warehose should have the immediate area cleared in a day or 2.